1. Field of the Invention
This invention relates generally to correcting the pixel placement of light beams in a raster output scanner. More particularly, this invention relates to adjusting a firing rate of two tangentially offset light beams so as to correct the pixel placement on the surface of the image receiving member.
2. Description of Related Art
Flying spot scanners (often referred to as raster output scanners or ROSs) conventionally have a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a fast scan direction while the recording medium is being advanced in the slow scan direction (the process direction). The beams scan the recording medium based on a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with the binary sample string, whereby the recording medium is exposed to the image represented by the samples as it is being scanned. Printers that sweep several beams simultaneously are referred to as multi-beam printers. Both ROS and multi-beam printer techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura, the subject matter of which is incorporated herein by reference.
High speed process color or multi-highlight color xerographic image output terminals typically simultaneously print multiple independently addressable raster lines at separate exposure stations. This is called multi-station printing. Conventional architectures for multi-station process color printers use a plurality of separate ROSs, usually four independent ROSs, one for each system color, for example, as illustrated in U.S. Pat. Nos. 4,847,642 and 4,903,067 to Murayama et al., the disclosures of which are incorporated herein by reference.
U.S. Pat. No. 5,243,359 to Tibor Fisli, the disclosure of which is incorporated herein by reference, discloses a ROS system suitable for deflecting multiple laser beams in a multi-station printer. A rotating polygon mirror simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams, having their largest divergent angles parallel to one another. The laser beams are subsequently separated by a plurality of optical filters and are directed onto their associated photoreceptors. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar optical path lengths for each beam. The lasers in U.S. Pat. No. 5,243,359 are arranged in the slow scan direction (i.e., sagittally offset). Diodes arranged in the slow scan direction must be arranged such that they are packed closely in a direction parallel to the polygon mirror's rotational axis to minimize beam characteristic deviations such as spot size, energy uniformity, bow and linearity. Thus, the laser diodes are kept as closely as possible (in the direction parallel to the polygon mirror's rotational axis) so the light beams strike nearly the same portion of the polygon mirror as possible.
U.S. Pat. No. 5,341,158 to James Appel et al., the disclosure of which is incorporated herein by reference, discloses a ROS system having laser beams tangentially offset in the fast scan direction (i.e., separated horizontally) to offset the diode spacing constraints of U.S. Pat. No. 5,243,359 to Fisli.
FIG. 1 illustrates a multi-station printer 10 similar to that disclosed in U.S. Pat. No. 5,341,158. In FIG. 1, the four laser beams, having different wavelengths, are reflected from the rotating polygon mirror 12. As the polygon mirror 12 rotates about its axis 13, the polygon mirror 12 simultaneously deflects the beams through a lens system 20 that focuses the beams and corrects for errors such as polygon angle error and wobble. The beams are then separated by optical filters 14, 16 and 18 and directed onto the photoreceptors 30, 32, 34 and 36 using mirrors 26. The multi-station printer 10 is preferably used for full color xerographic printing or copying. As is well known, each laser beam produces a latent image on its associated photoreceptor 30, 32, 34 and 36 that corresponds to a system color that will be transferred onto a recording medium (not shown), such as, for example, plain paper or an intermediate belt (not shown). The photoreceptors 30, 32, 34 and 36 can be, for example, belts or drums.
FIG. 2 illustrates two optical input channels used to scan two tangentially offset laser beams 41 and 51 across the polygon mirror 12. For example, laser diode 40 emits a laser beam 41 that passes through a collimator 42 and lens 44 before being reflected from the polygon mirror 12. Similarly, laser diode 50 emits a laser beam 51 that passes through a collimator 52 and lens 54 before being reflected from the rotating polygon mirror 12. Mirrors 46 and 56 may additionally be provided to direct the light beams 41 and 51 onto the rotating polygon mirror 12. As is clearly seen in FIG. 2, the two light beams 41 and 51 are tangentially offset in the fast scan direction.
Upon being reflected from the polygon mirror 12, each of the beams 41 and 51 passes through an F.sub..theta. lens 22 and an anamorphic element 24 onto the surface of a photoreceptor (not shown in FIG. 2). As is well known in the art, the f.sub..theta. lens 22 corrects for scan linearity in a well known manner. The anamorphic element 24 generally provides only limited power for the sagittal focusing of the beam. For ease of illustration, the post polygon optics including lens 22 and element 24 only illustrates one beam. However, both beams 41 and 51 actually pass through the post polygon optics. Additionally, the lens 22 and element 24 of FIG. 2 may correspond to the lens system 20 of FIG. 1. The present invention is preferably used in a multi-spot system where two beams are tangentially offset as in FIG. 2.
Polygon scanners such as that described above with reference to FIGS. 1 and 2 are well known in the art and are described, for example, in "Laser Scanning for Electronic Printing," Proceedings of the IEEE, Vol. 70, No. 6, June 1982 by John C. Urbach et al., the disclosure of which is incorporated herein by reference. Other optical polygon scanner embodiments are similarly known and are within the scope of this invention.
FIG. 3 illustrates multiple laser diodes 40, 50, 60 and 70 formed on a single chip 38. As is shown in FIG. 3, each of the laser diodes 40, 50, 60 and 70 is tangentially offset in the fast scan direction (i.e., the X direction). The tangentially offset laser diodes are also described, for example, in U.S. Pat. No. 5,341,158 to Appel et al.
When separate laser beams are tangentially offset as in U.S. Pat. No. 5,341,158 to Appel et al., each of the laser beams strikes the polygon mirror 12 at a different angle. The present invention is preferably applicable to a much greater input angle between laser beams such as illustrated in FIG. 2 by the 10.degree. angle offset between laser beams 41 and 51 prior to striking the current facet of the polygon mirror 12. FIG. 4 illustrates another embodiment of tangentially offset laser diodes 40 and 50. In FIG. 4, the laser diodes 40 and 50 are both sagittally offset (in the slow scan direction) and tangentially offset (in the fast scan direction). This is preferably used to create a pitch separation between scanning spots on a photoreceptor when multi-spot printing is used. Embodiments of the present invention will generally be described with respect to the tangentially and sagitally offset beams of FIG. 4. However, the description of this embodiment is in no way limiting.
FIGS. 5A-5B illustrate one preferred embodiment of how the ROS modulates the laser beams 41 and 51 as they are scanned along the fast scan direction. As is well known, the ROS may include a start of scan (SOS) detector 90 (such as a slit detector) used to control the start of scan of the laser beams 41 and 51 as they enter into the fast scan region. As is well known in the art, the SOS detector 90 generates a signal as a spot passes through it. The signal is sent to an electronic subsystem (ESS). The ESS controls the formatting and the flow of information (data stream) into the light modulator of the ROS. When laser diodes are used, the current input to the laser "driver" electronics is controlled. At the arrival of an SOS signal, the ESS clocks out a complete line of raster data. FIG. 5A shows each laser beam 41 and 51 having not yet reached the start of scan detector 90. Therefore, the beams 41 and 51 are not modulated onto the surface of a respective photoreceptor. FIG. 5B shows the start of scan detector 90 detecting the beam 51 but not detecting the beam 41 because of the tangential separation between the beams 41 and 51. The amount of separation between the two beams 41 and 51 is determined by the amount of separation between the laser diodes 40 and 50. For example, in FIGS. 5A-5B, the beams 41 and 51 are sagittally offset as indicated by distance A and are tangentially offset as indicated by distance B.
When beam 51 arrives at the SOS detector 90, the ESS clocks a complete line of raster data for that beam. This raster data generally includes a margin and the respective print data to be printed by the beam 51. The margin represents a delay of the laser diode 50 until the beam 51 is actually modulated onto the photoreceptor. As shown in FIG. 5B, the beam 41 has not yet reached the SOS detector 90. The delay between the beams 41 and 51 can be determined knowing the separation between the laser diodes 40 and 50 or by experimentation. As is well known in the art, once the beam 51 passes through the SOS detector 90, the necessary delay of the beam 41 is computed. This is accomplished by causing a greater delay for the beam 41 than for the beam 51 so that both beams can be modulated onto the photoreceptor at similar pixel positions in the fast scan direction. Thus, the margin for the beam 41 is greater than the margin for the beam 51.
FIG. 6 shows a pair of vertical lines 110 and 112 extending in a process direction. To produce the lines 110 and 112, the pixels of each of the beams 41 and 51 must be precisely positioned in the fast scan direction to properly modulate onto the photoreceptor. The light beam 51 will scan across the scan line 53 and be appropriately modulated by the ROS. The light beam 41 will be scanned across the scan line 43 and will also be independently modulated by the ROS.
As is apparent to those skilled in the art, when vertical lines are desired to be printed, the pixel placement of each of the light beams 41 and 51 must be precisely controlled. For example, if one of the light beams is modulated so that a pixel is misplaced (in terms of the fast-scan position) from the desired location by any amount, then the vertical lines will show jagged edges. Therefore, if both beams are scanned with similar scan linearity, then the prior art methods would compute the necessary delays between each of the beams 41 and 51 so that the vertical lines will be correctly modulated onto the photoreceptor to form the lines 110 and 112 with jagged edges. For example, if the vertical lines 110 and 112 are desired to be printed exactly one inch apart then the spot velocity for each of the beams 41 and 51 must be correct and perfectly linear.
However, when beams are tangentially offset as in FIG. 2, the scan linearity for each of the beams 41 and 51 differs as the beams are scanned across the scan lines 43 and 53. The scan linearity differences result from the light beams passing through different portions of the post-polygon optics. This is caused by the large tangential separation of the light beams in FIG. 2.
Therefore, the ROS computes the respective margins so that the light beams 41 and 51 are correctly modulated to a desired position to form the first vertical line 110 and the second vertical line 112. However, as explained above, because each of the beams may have a different scan linearity, the exact modulation of the beams 41 and 51 may not occur on the vertical lines 110 and 112 and a visual displeasing image may be printed. In other words, when the above described delay is implemented, all the scan lines can start in the required position. However, from then on the position of the pixels will not be correct because the spots will have different velocities due to the differences in the angular input of the beams.